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United States Patent |
5,632,900
|
Jevec
,   et al.
|
May 27, 1997
|
Wet oxidation method of treating chelate bearing waste solutions
Abstract
A method for treating a chelate bearing waste solution employs a two-step
process which includes elevating the pH of the solution to a pH greater
than 10 and precipitating the metal as the hydroxide out of solution,
followed by treating the resultant solution with an oxidant to destroy any
remaining free chelate.
Inventors:
|
Jevec; John M. (Uniontown, OH);
Lenore; Carl J. (North Benton, OH)
|
Assignee:
|
The Babcock & Wilcox Company (New Orleans, LA)
|
Appl. No.:
|
424834 |
Filed:
|
April 19, 1995 |
Current U.S. Class: |
210/721; 210/724; 210/761; 210/912 |
Intern'l Class: |
C02F 001/72 |
Field of Search: |
210/721,722,724,758,759,761,762,912-914
|
References Cited
U.S. Patent Documents
4419246 | Dec., 1983 | Jayawant | 210/721.
|
4512900 | Apr., 1985 | Macur et al. | 210/912.
|
4629570 | Dec., 1986 | Kennedy, Jr. | 210/912.
|
5160631 | Nov., 1992 | Frost et al. | 210/724.
|
5178772 | Jan., 1993 | Daley et al. | 210/721.
|
Other References
Evans, D.W. et al., "Treatment and Disposal of Steam Generator and Heat
Exchanger Chemical Cleaning Wastes", Ontario Hydro Research, pp. 1-12,
admitted art.
Beller, John, "Supercritical Water Oxidation Program", admitted art.
"Pilot Plant Evaluation of the Mobile Wet Oxidation Process", Finger, S. et
al.
Duratek Corp., Atwood, Allen, Bechtel Corp. admitted art.
|
Primary Examiner: Wyse; Thomas G.
Attorney, Agent or Firm: Kalka; Daniel S., Edwards; Robert J.
Claims
We claim:
1. A method for treating a chelate metal bearing waste solution from a
chemical cleaning solution for a steam generator, comprising the steps of:
elevating the pH of the chelate bearing waste solution from a chemical
cleaning solution for a steam generator to a pH of greater than about 10.0
for separating a metal from the chelate bearing waste solution as a
precipitate;
filtering the chelate bearing waste solution after precipitation to remove
the metal precipitate; and
oxidizing the chelate bearing waste solution to destroy any remaining free
chelates.
2. A method as recited in claim 1, wherein the oxidizing step further
comprises the step of treating the chelate bearing waste solution with an
oxidant.
3. A method as recited in claim 1, further comprising the step of adjusting
a pH of the chelate bearing waste solution after the filtering step to a
pH near neutral; and
then oxidizing the resultant solution to destroy any remaining free
chelates.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a process for chemical waste
disposal, and more particularly to a wet oxidation process for disposal of
a chelate waste solution.
2. Description of the Related Art
Chelates are used in both fossil and nuclear steam generator chemical
cleaning processes. Waste disposal from a fossil unit cleaning is normally
sent off site for treatment and disposal. Nuclear cleaning wastes are
generally slightly contaminated with radionuclides and as such require
disposal at an approved nuclear waste disposal site.
Due to the high disposal costs at approved nuclear waste disposal sites,
cleaning wastes are currently evaporated to less than 85% of the original
volume. The resultant concentrate is then solidified and sent for burial.
While this is sufficient for the present, it is anticipated in the near
future that the disposal of chelates at approved nuclear waste sites will
be discontinued. The chelates will have to be removed from the chemical
cleaning waste prior to disposal.
U.S. Pat. No. 5,225,087 describes a recovery process of EDTA from steam
generator cleaning solutions. U.S. Pat. No. 4,780,238 relates to natural
chelating agents for radio nuclide decorporation. U.S. Pat. No. 4,681,705
describes a method of decontaminating mixtures of radioactively
contaminated liquids. U.S. Pat. No. 4,587,043 also describes the
decontamination of metal surfaces in nuclear power reactors. U.S. Pat. No.
4,880,595 relates to a process and apparatus for cleaning nuclear reactor
cooling water. Other patents related to this subject matter include U.S.
Pat. Nos. 5,115,036; 4,578,162; and 4,629,570.
Traditional wet oxidation processes are very effective in the destruction
of free chelates. A free chelate is defined as those chelates that have
not formed a complex with a metal ion. In a chemical cleaning solution,
the majority of the chelates are in the complex state. This makes the
chelate difficult to destroy using the traditional wet oxidation methods.
Accordingly, there is a need for a process which allows the safe disposal
of chelate. Preferably, the method would separate the chelate from the
metal complex to a free state where it could then be treated with a wet
oxidation method to achieve a destruction of the chelate to 99.9% or
greater.
SUMMARY OF THE INVENTION
The present invention is directed to the above-mentioned problems with the
prior art as well as others by providing a two-step process which results
in the destruction of chelates in chemical cleaning solutions. The first
step in the process of the present invention is to separate the chelate
from the metal complex. After achieving this, the metal ions are
precipitated and filtered from the solution. The resultant solution
contains only free chelates which are then treated with wet oxidation
methods to achieve a 99.9% or greater destruction. In the initial step,
the pH of the solution is elevated with the addition of a base to a pH of
10 or greater. This is sufficient to separate the metal from the chelate
complex and precipitate the metal as a hydroxide. After precipitation, the
solution is filtered to remove the metals. The resultant solution is then
treated with an oxidant to destroy any remaining free chelates.
One aspect of the present invention is to provide a process for treatment
and disposal of waste solutions containing chelates.
A second aspect of the present invention is a two-step process for treating
chelate bearing waste solutions.
The various features of novelty which characterize the invention are
pointed out with particularity in the claims annexed to and forming a part
of this disclosure. For a better understanding of the invention, its
operating advantages and specific objects attained by its uses, reference
is made to the descriptive matter in which the preferred embodiment of the
invention are illustrated with examples.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention resides in a two-step process for treating chemical
cleaning solutions containing chelates. The first step in the process is
to elevate the pH of the solution with the addition of a base, such as
sodium hydroxide (NaOH), to a pH of about 10, preferably 10.5 or greater,
which is usually sufficient to separate the metal ion from the chelate
complex and precipitate the metal as a hydroxide. Some metals are
particularly difficult to precipitate, for example copper. In those cases,
it is necessary to use additional additives to force the precipitation of
the metal ions. Suitable additives would include the following additives;
calcium hydroxide, calcium oxide and dialkyldithiocarbamate. The
concentrations of additives is determined by the concentration of metal
ions to be removed.
After precipitation, the solution is filtered to remove the metals. The
remaining solution is treated with an oxidant like hydrogen peroxide
(H.sub.2 O.sub.2) to destroy any remaining free chelates. It is
envisionable that any traditional wet oxidation method can be used at this
stage to oxidize the chelate to make them suitable for disposal. This
second step of the process proceeds more efficiently if the pH of the
solution is dropped back to or near neutral (about pH=7.0). This does not
add any unnecessary steps to the process since the resultant pH of the
solution needs to be adjusted to near neutral prior to discharge or
disposal. Advantageously, the method of the present invention is a low
temperature application that is applied at ambient temperature. The
chelate destruction reactions are exothermic so that temperatures
approaching the boiling point, i.e., 100.degree. C. (212.degree. F.) often
occur particularly when there are large quantities of chelate present.
After this destruction, the resultant solution is free of chelates and
metal ions.
The chelate destruction process of the present invention results in a
solution that is acceptable for discharge to a local sewer system after
proper verification of results. This greatly reduces the burial costs
associated with disposal of steam generator chemical cleaning wastes since
a premium is paid for wastes containing chelates. In addition, the burial
of chelate bearing nuclear cleaning wastes is anticipated to be severely
restricted in the future.
EXAMPLES
A three test matrix was evaluated in this study. Each test was run with 500
ml of solvent (initial total EDTA of 150 g/l). The first test solution was
pH adjusted to 10.5 with sodium hydroxide. The solution was filtered
through a Whatman #41 filter paper then through a 0.45 micron Millipore
membrane. The filtration process took over 1 hour to complete due to the
fine consistency of the ferric hydroxide sludge. The filtrate was placed
in a one liter beaker, stirred continuously, and 100 ml of 50% hydrogen
peroxide added over a period of 15 minutes. The peroxide addition resulted
in severe outgassing of the solution and the exothermic reaction raised
the solution temperature to about 205.degree.-210.degree. F. almost
immediately. The peroxide was added with a burette near the bottom of the
beaker. After 24 hours the solution was analyzed for total EDTA content.
The pH of the second 500 ml solution was increased to about 11.5.
Precipitated ferric hydroxide was filtered in the same manner described
above. Again, it took a considerable length of time to filter the
ironladen solution. 100 ml of 50% hydrogen peroxide was added to the
filtered solution. This time there was not obvious outgassing as the
peroxide was added nor was there evidence of an exothermic reaction.
However, after standing idle for a period of 2.5 hours a violent reaction
took place. Rapid outgassing and an extreme exothermic reaction caused the
solution to explode out of the beaker. This reaction took place
instantaneously and only approximately 50ml remained in the beaker (for
analysis) after the reaction occurred.
The pH of the third solution was increased to 11.5 the iron filtered off
and then the solution was readjusted back to 7.0. Hydrogen peroxide was
then added to the solution at the neutral pH. Peroxide addition resulted
in solution outgassing and a rapid exothermic reaction. This solution was
allowed to stand for 24 hours and then checked for total EDTA.
The total EDTA values for the three test solutions 10 are presented below:
______________________________________
TEST 1 (pH 10.5)
18.8 g/l Total EDTA
TEST 2 (pH 11.5)
17.6 g/l Total EDTA
TEST 3 (pH 7.0)
0.19 g/l Total EDTA
______________________________________
These test results basically show that once the EDTA-iron bond is broken,
the EDTA is readily decomposed by the presence of hydrogen peroxide. In
fact, only 66% of the stoichiometric amount of 50% peroxide was added to
achieve the greater than 99% reduction in EDTA in Test 3.
The high pH-wet oxidation method basically removes all iron and EDTA from
solution. However, cobalt and cesium would still be present in an actual
spent nuclear cleaning solvent.
Although specific embodiments of the invention have been shown and
described in detail to illustrate the application of the principles of the
invention, it is understood that the invention may be embodied otherwise
without departing from such principles.
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